26 THE FLIGHT OF BIRDS 



proportion as the glider makes a smaller and smaller 

 angle with the horizon, the point at which the air 

 acts will be progressively nearer to the front margin, 

 and this will obviously tend to prevent a dive 

 downward. 



Fig. 12 

 Diagrams showing the shifting of the centre of pressure. 



This point requires some explanation. Take as 

 an illustration a sailing boat when it is tacking. 

 When it sails close to the wind, the air which strikes 

 against the forepart of the sail is deflected from its 

 course, and rushing towards the stern of the boat 

 forms a buffer which shields the rest of the sail from 

 the air, which would otherwise have impinged upon it. 

 The drawing of a boat tacking, on page 8, and 

 the accompanying diagram, will make this clear. 

 The principle is called the law of Avanzini. The 

 smaller the angle made by the wind with the sail, 

 the nearer the point of impact approaches to the 

 front edge. This is true no less of the slightly curved 

 surfaces of aeroplanes. As the angle of incline to 

 the horizontal is more and more reduced, the wind 

 acts at a point closer and closer to the front edge, 

 and thus the aeroplane may, possibly, correct the 

 dangerous tendency automatically, without the 

 aviator having to make readjustments. 



To illustrate this principle I have made experi- 

 ments with a catapult which, working within 

 runners, threw pieces of cardboard horizontally, 



